One of the curbs on the development of proton-exchange membrane low-temperature fuel cells (PEMFC) lies in the need to use materials that are active enough to catalyse oxygen reduction reactions (ORR) and hydrogen oxidation reactions (HOR).
At the present time, the best catalyst that has been found for these two reactions is platinum and some of its alloys with transition metals (Ru, Ni, Co, etc.). To obtain optimum performance under operating conditions that are compatible with common applications, it is currently necessary to fill the electrodes with relatively large amounts of noble metal (0.1-0.3 mg/cm2).
The cost of the noble metal alone used in fuel cells is currently estimated at 50% of the cost of the stack and between 20% and 25% of the total cost of the fuel cell system.
Consequently, reducing the amount of platinum, while at the same time keeping the performance constant for cells, is a major challenge in research and development on catalysts for the cathode of fuel cells. In the face of this problem, one of the strategies consists in placing the platinum only in zones where its activity will be maximal. Since the electrochemical reactions concerned simultaneously involve reagent gases, electrons and protons, these zones have at least one ionic contact and one electrical contact. In order for a catalytic site to be fully effective, it also needs to be accessible to the reagent gas (hydrogen or oxygen). One of the options for satisfying these two criteria consists in using an electrochemical deposit, also known as an electroplating. The method consists in applying a galvanic signal or a signal at controlled potential in order to reduce the metal salt (platinum in ionic form PtCl62− for example) to metal particles deposited on the electrode which serves as substrate for the deposit.
The reaction balance for the deposition of platinum is written as follows:2H+,PtCl62−+4e−→Pt+2HCl+4Cl−
Although the technique has been used for many years, as witnessed by the studies described in Vilimbi N. R. K., Anderson E. B., and Taylor E. J. U.S. Pat. No. 5,084,144, or in: E. J. Taylor, E. B. Anderson, N. R. K. Vilambi, Journal of the Electrochemical Society 1992, 139 L45-L46., the method has not ceased to be studied to improve the performance of these electrodes prepared by electroplating. Among the major studies on the subject, mention may be made of the following articles: S. M. Ayyadurai, Y. S. Choi, P. Ganesan, S. P. Kumaraguru, B. N. Popov, Journal of the Electrochemical Society 2007, 154 B1063-B1073, H. Kim, B. N. Popov, Electrochemical and Solid State Letters 2004, 7 A71-A74, in which the electrodes charged with 0.32 mgPt/cm2 made it possible to achieve a current density of 900 mA/cm2 at 700 mV by running the cell with H2—O2 at 80° C. under atmospheric pressure.
However, in many cases, it appears that the size of the particles of these electrodes is at most 10 times coarser than those prepared via the standard chemical method. This phenomenon is very detrimental to the catalytic activity, since the reactions involved are reactions that take place at the surface of the metal particles and at constant charge, the developed surface of these particles is proportionately smaller the larger the diameter of the particles. There is thus a risk of losing the gain in efficacy associated with the optimum placing of the particles if they become too coarse.
One of the major objects in the optimization of the electroplating technique is thus to manage to reduce the size of the particles to diameters that are comparable to those obtained via chemical or physical synthesis, i.e. less than 10 nm. Admittedly, the prior art features studies in which electrodes have been prepared by electroplating with particles whose size is of the order of a few nanometres, however, in many cases, the charge is very low and does not make it possible to be used as a fuel cell cathode (a charge of the order of 150 to 200 μgPt/cm2 is necessary for optimum functioning). Another method for reducing the particle size consists in adding a viscous agent to the electrolytic bath in order to curb the diffusion of the metal ions, which makes it possible to significantly reduce the particle size (by 5 to 30 nm) as described in the following articles: Ayyadurai, Y. S. Choi, P. Ganesan, S. P. Kumaraguru, B. N. Popov, Journal of the Electrochemical Society 2007, 154 B1063-B1073 or W. Zhu, D. Ku, J. P. Zheng, Z. Liang, B. Wang, C. Zhang, S. Walsh, G. Au, E. J. Plichta, Electrochimica Acta 2010, 55 2555-2560.
This alternative gives very interesting results but poses the problem of cleaning these viscous agents (glycerol or ethylene glycol) from the electrodes. Specifically, their presence in high concentration in the diffusion layer of the electrode not only curbs the diffusion of the Pt ions during the electroplating, but also the access of oxygen to the active sites, and thus contributes towards substantially reducing the performance of the cell.